In the manufacture of a communications cable, a core comprising a plurality of insulated conductors is enclosed in a strip of plastic material, which is referred to as a core wrap. Subsequently, the core wrap is enclosed by other components of a sheath system such as, for example, a metallic shield and a plastic jacket. The core wrap insulates the core from the metallic shield which is designed to protect the cable core from electrical disturbances. The core wrap and the shield, which often times is corrugated, are formed from continuous strips which are wrapped longitudinally with overlapped seams about an advancing cable core.
In order to preserve the transmission qualities of a communications cable, it becomes necessary to prevent the ingress of moisture into the multiconductor core of the cable. This may be accomplished by introducing a pressurized gas. In an increasingly popular technique, the cable core is filled with a waterproofing material and wrapped with a strip of plastic material after which a metallic tape is wrapped about the core to form a shield and has its outer surface flooded with another waterproofing material. Typically, a waterproofing material such as one designated Flexgel.TM. filling material, is used to fill the core and an atactic material is used to flood the shield. Flexgel is a trademark of AT&T Technologies, Inc. Such a filling material is disclosed in U.S. Pat. No. 4,176,240 which issued on Nov. 27, 1979 in the name of Ralph Sabia. The last-described process produces what is referred to as a filled cable and avoids the necessity of pressurizing the cable.
For some filled cables, a dual metallic shield system is used. An inner shield which is made of a material having a relatively high electrical conductivity is used to dissipate stray electomagnetic currents and lightning. Formed about the inner shield which may comprise aluminum, for example, and which has its outer surface flooded with an atactic material is an outer shield which is made of a material such as steel having a relatively high modulus of elasticity. The outer shield provides suitable mechanical strength for the cable. Such a cable which also includes a plastic jacket is commonly referred to as an ASP cable. Each of the shields typically is formed with a longitudinal seam which at least for the outer shield usually is overlapped.
It is not uncommon to bond the plastic jacket to the outer surface of the outer shield. By bonding the plastic jacket to the outer shield, which generally is corrugated, it has been found that the resistance of the cable to moisture diffusion is increased substantially. Also, it provides mechanical strength to resist buckling, crushing and scuffing. Further, if the jacket is not bonded to an adjacent shield, the pulling of the cable into an underground duct more often than not will cause a separation of the jacket from the shield.
There is a need to seal the longitudinal overlapped seam of the outer shield. The sealing of the seam further reduces the probability of moisture ingress. Secondly, it prevents the escape of waterproofing material from the interface between the inner and outer shields to the external surface of the outer shield. Should any such material become deposited on the outer surface of the outer shield, bonding of the jacket to the shield is impaired. As a result, the jacket may exhibit irregularities or easily become separated from the outer shield when the cable is pulled into a duct. Further, a sealed seam is helpful in preventing the overlying edge portion of the outer shield from protruding into the jacket and weakening the plastic.
In order to insure that moisture is kept out of the core, the core wrap also should be provided with a waterproofing material. The inner surface of the core wrap material contacts waterproofing material inasmuch as it is wrapped about the filled core. As the strip of plastic is wrapped about the core, the waterproofing material on the inner surface tends to be flowed toward the seam to seal it. However, it would not be wise to depend on portions of the core filling material to be flowed into the seam area as that could result in voids between the core and the inner surface of the core wrap. Therefore, a technique is required which will provide the inner surface of the core wrap with sufficient waterproofing material to seal the seam. Also, the application of the waterproofing material to the outer surface of the strip of plastic material must be accomplished in a controlled manner. Otherwise, for example, some of the waterproofing material on the outer surface of the core wrap could seep through the seam of the aluminum shield which is wrapped subsequently or simultaneously about the core and contaminate the atactic material which is applied to the outer surface of the aluminum shield.
The prior art appears to be devoid of methods and apparatus which are suitable for controllably applying a waterproofing material to an advancing strip of plastic material which is destined to enclose a cable core. What is needed are methods and apparatus for applying waterproofing material to a plastic strip in any one of several sheath arrangements.